Tumor specific molecules to aid in better diagnosis and treatment of human and animal cancer have been sought since the last century. Hard evidence of tumor-specific substances, based on molecular structural data, has been difficult to provide in most types of human cancer except those based on virally-induced cancer and involving molecular structures specified by the virus genome. There have been extremely few examples of tumor-specific molecules based on novel molecular structures. In the case of malignant human gliomas and other tumors potentially associated with amplification or changes in the epidermal growth factor receptor molecule, such as carcinoma of the breast and other human carcinomas, there have been no unequivocal demonstrations of structurally altered molecules with unique sequences.
The epidermal growth factor receptor (EGFR) is the 170 kilodalton membrane glycoprotein product of the proto-oncogene c-erb B. The sequence of the EGFR gene is known (Ullrich et al., 1984). The EGFR gene is the cellular homolog of the erb B oncogene originally identified in avian erythroblastasis viruses (Downward et al., 1984; Ullrich, et al. 1984). Activation of this oncogene by gene amplification has been observed in a variety of human tumors (Haley et al., 1987a), and in particular, those of glial origin (Libermann et al., 1985; Wong et al., 1987; Yamazaki et al., 1988; Malden et al., 1988).
One major difference between v-erb B oncogenes and the normal EGFR gene is that the viral oncogenes are amino-truncated versions of the normal receptor; they lack most of the extracytoplasmic domain but retain the transmembrane and tyrosine kinase domains (Fung et al., 1984; Yamamoto et al., 1983, Nilsen et al., 1985; Gammett et al., 1986). This results in a protein that is unable to bind epidermal growth factor (EGF) but can still phosphorylate other substrates (Gilmore et al., 1985; Kris et al., 1985), and has led to speculation that the v-erb B proteins are oncogenic because the kinase domain is unregulated and constitutively active (Downward et al., 1984).
A variety of genetic alterations can occur in viral erb B oncogenes, e.g. amino acid substitutions and deletions in the carboxy terminus of the gene. Available evidence, however, argues that the amino truncation is critical to carcinogenesis. Amino truncations are a feature of all v-erb B oncogenes, including those that arise by promoter insertion or retroviral tranduction (Nilsen et al., 1985; Gammett et al., 1986).
In contrast, carboxy-terminal deletions appear to be associated only with tumors that arise through retroviral transduction and seem to determine host range and tumor type specificity (Gammett et al., 1986; Raines et al., 1985). Transfection experiments with amino-truncated avian c-erb B genes or chimeric viral oncogene-human EGF receptors demonstrates that this deletion is sufficient alone to create a transforming protein (Pelley et al., 1988; Wells et al., 1988).
Amplification of the EGFR gene occurs in 40% of malignant human gliomas (Libermann et al., 1985; Wong et al., 1987). Rearrangement of the receptor gene is evident in many of the tumors with gene amplification. The structural alterations seem to preferentially affect the amino terminal half of the gene (Yamazaki et al., 1988; Malden et al., 1988), but the nature of the rearrangements has not been precisely characterized in any tumor.
Size variant EGFR genes and amplification have been reported in several human cancers. (Humphrey et al., 1988; Bigner et al., 1988; Wong et al., 1987; and Humphrey et al., 1989) There has been no determination, however, of the molecular basis for the altered EGFR molecules in cells. A determination of the genetic changes responsible for these tumors would present a significant step forward in the treatment and diagnosis of human carcinoma.
It would be desirable to have unique gene and peptide sequences for glioma EGFR. It would also be desirable to have a synthetic peptide against which monoclonal or polyclonal antibodies could be produced which demonstrate specificity against mutant EGFR.